In electrophotographic apparatuses such as copying machines and laser printers, the surface potential of a photoreceptor largely varies with environmental changes. The surface potential of the photoreceptor is gradually lowered by mechanical stress. This is because mechanical stress, such as abrasion by a cleaning blade, is accumulatively applied to the photoreceptor during repeatedly preformed image forming operations, and the thickness of a photosensitive layer on the photoreceptor is reduced. Such changes in the surface potential of the photoreceptor greatly affect the image quality, for example, cause a decrease in the image density.
For example, in a device disclosed in Japanese Publication for Examined Patent Application No. 29502/1986, an electrostatic latent image formed on a photoreceptor is first read. Then, the formation of electrostatic latent image is controlled based on the results to give stability to the image quality. There is also a device provided with an optical sensor for detecting the optical density of a reference toner image called a toner patch. In the device of this type, a charging voltage and a discharge voltage are controlled during electrophotographic processes so that the optical density of a toner patch formed on a photoreceptor to be detected by an optical sensor is equal to a reference value determined to achieve optimal image quality.
In a high-speed copying machine, a pre-cleaning charger is disposed before a cleaning device for removing toner remaining on the photoreceptor after the transfer process. The pre-cleaning charger removes unwanted charges remaining on the photoreceptor. When an output of the pre-cleaning charger exceeds its optimum value, it causes the photoreceptor to be fatigued and insufficient cleaning of toner charged to the opposite polarity. To prevent such problems, the output of the pre-cleaning charger device is arranged to be within a predetermined range.
In copying machines, however, the image quality is adjusted simply by controlling machine function such as charging, exposure, and developing without controlling a change in transfer efficiency caused by degradation of a developer and environmental changes. Consequently, even when the image quality is adjusted, if there is a change in the transfer efficiency, it affects the quality of the resulting image.
Moreover, in a conventional copying machine, when the transfer efficiency is lowered due to a change in the quality of transfer sheets caused by the deterioration of a transfer device and environmental changes, the amount of toner remaining of the photoreceptor after the transfer process increases, causing degradation of cleaning. However, even in a copying machine having a pre-cleaning charger disposed before a cleaning device, degradation of cleaning due to a change in the transfer efficiency is not taken into account for the control of the output of the pre-cleaning charger.